Defining an adaptive waveform strategy for designing an Airborne Ground Penetrating Radar needs to accurately understand all the phenomena occurring when a High Range Radar waveform penetrates inside the soil. In this paper, all the phenomena affecting the dielectric constants and their variations within the soil have been detailed. Then, we focus our attention on the pulse envelope modulation due to the Air-Soil interface crossing, that induces an effective bandwidth reduction and then defocusing effects. In fact, we assume a dispersive and lossy medium, a random rough interface and a frequency modulated continuous wave (radar scheme), with a possible pulse envelop modulation. Our results show that the zeroth and first orders have to be considered jointly for a realistic simulation of the electromagnetic propagation in particular for rough surfaces as well as the depolarization effects. It also shows that the bandwidth variation is strong, for the zeroth as for the first order, and an adaptive waveform has to take into account the defocusing effects due the scattering coefficient variation (even for smooth and dry surfaces). Our calculations also give the admissible EMW losses into the soil to detect a buried object with regards to the interface backscattering by comparing the first order (surface backscattering) and the squared zeroth order (double crossing of the interface).